I edited the .htaccess file and uploaded it, but it didn't work so I have contacted my host. I was curious though, if I make a new user account through the admincp and then go and try to edit it, it will let me edit that account. Just not any older ones I guess before today. Does that still sound like it could be mod_security?
EDIT: Nevermind, my host has fixed it. Thanks so much for your help.